What audio interface are you using? It may depend on the manufacturers implementation of their ASIO driver. Others have had success sharing unused ASIO channels with Audition, I believe.
It does as I've used an interface where it didn't matter. BUT cubase doesn't have any issues releasing the asio channels with my audio card so it is obviously an issue with Audition and most other audio apps. It seems they try and keep all the asio channels to themselves and not play nice. With audition open I can't even play normal sound when using asio. For example, if I try to play a movie in GOM, WMP, or VLC I will not get any sound until I shut down Audition. Yet I can run VLC and GOM at the same time and get sound from both. (they are not using asio)
Again, since cubase works fine it is an issue with the way Audition handles the asio channels... simple as that. I have Audition setup for it's own special asio channel that I tell cubase to ignore and it does. I also give cubase it's own asio channel and "disable" it in Audition. Cubase listens, Audition does. Virtually ever other aiso application that I have tried does things similar to Audition and I believe it is simply a lack of releasing the unused/unneeded channels(well, until you shut down the program).